1. Field of the Invention
The present invention relates to an ultrasound Doppler detection method with Golay coded excitation and more particularly, relates to an ultrasound Doppler detection method with Golay coded excitation by a filter filtering out waves in a Doppler frequency domain of a slow-time.
2. Description
The technique of generating images by means of ultrasound has been widely adopted in biomedical applications. Compared with other medical imaging systems such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI) and nuclear medicine imaging utilized in clinic, ultrasonic imaging has advantages of cost effectiveness, non-invasiveness, no ionizing radiation, real-time imaging capability, high spatial resolution (less than 1 millimeter), portability, flow estimation ability, etc. Thus, ultrasound imaging has been commonly applied to clinical diagnosis in several medical categories.
Ultrasonic Doppler detection system is utilized for imaging, wherein the Doppler effect is utilized to determine whether a certain structure (usually blood flow) moves toward or away from a probe, to calculate its relative speed, and to determine its direction and speed for imaging by calculating a frequency drift of the volume of a partial sample (e.g. jet blood flow above the heart valve). According to said ultrasonic Doppler detection system, sinusoidal waves of ultrasound are transmitted and received for imaging.
However, since ultrasound is mechanical wave, sound pressure may generate heat and cavitation effects damaging human body. Besides, the amplitude of ultrasound is so great that nonlinear phenomenon is easily generated, which affects the quality of imaging. Thus, in medical applications of ultrasound, the amplitude of sound pressure of ultrasound is limited within the regulation of Medical Index. As a result, there are disadvantages such as low Signal to Noise Ratio (SNR) and low transmittance in ultrasound Doppler detection system.
The technology of code excitation is utilized for improving the limits of SNR of ultrasound Doppler detection system by means of phase modulation and frequency modulation. Coded waves are designed with longer transmission time and averagely low sound pressure. A result of short and high sound pressure is obtained after the coded waves are received, decoded and compressed so that the SNR and resolution of the Doppler detection are improved. There are two parts in the coded waves after compression, mainlobe and sidelobe. Sidelobe will cause artifacts and thus will affect the result of the Doppler detection system with the code excitation.
Phase encoding is utilized in Golay code of said code excitation. Phase encoding has advantages such as improving the energy of the mainlobe and completely inhibiting signals of sidelobe. On transmission end, a pair of complementary coded signals is transmitted. The signals are received at the receiving end and compressed via corresponding matched filter so as to remove sidelobe and enhance the strength of mainlobe. Compared with other coded waves, Golay code is relatively easy in terms of design and process of hardware, which can effectively reduce production cost and is beneficial for manufacturing smaller instruments.
Theoretically speaking, sidelobe signals should be completely inhibited by means of Golay code, which sums up complementary codes; however, practically speaking, when observing moving objects, the ability of summing up complementary codes for inhibiting sidelobe signals is greatly weakened and residual sidelobe signals caused will affect the precision of the Doppler detection method. Therefore, it can only be applied for slowly moving instead of fast moving objects such as high speed moving blood flow. As a result, Golay code is usually deemed as not suitable for Doppler detections.